The present invention relates to a semiconductor structure, and more specifically, to a semiconductor structure and method for obtaining tunable threshold voltages in high-k metal gate stacks and complementary metal oxide semiconductor (CMOS) devices.
The use of silicon germanium (SiGe) as the channel of a p-FET device has been shown to reduce the flatband and threshold voltage to the p-FET band edge. This shift in the flatband and threshold voltage has been associated with the band-gap modulation of SiGe with the incorporation of germanium (Ge). However, the voltage shifts that are obtained in semiconductor devices are not entirely consistent with the sole consideration of band-gap narrowing.
Further, there is an undesirable shift in the threshold voltage of an n-FET device which cannot be explained due to valence band off-set by the use of SiGe. The shift in the threshold voltage of the n-FET device may be related to the presence of germanium (Ge) near the high-k dielectric and in its subsequent incorporation in the n-FET gate stack.